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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 20 |
| 5 | 80 |
| 6 | 366 |
| 7 | 1904 |
| 8 | 11080 |
| 9 | 71424 |
| 10 | 505210 |
| 11 | 3891712 |
| 12 | 32433180 |
| 13 | 290787328 |
| 14 | 2791053734 |
| 15 | 28556359680 |
| 16 | 310264194320 |
| 17 | 3567710830592 |
| 18 | 43287834157938 |
| 19 | 552688817143808 |
| 20 | 7407423764750500 |
| 21 | 103981459115606016 |
| 22 | 1525675236649033822 |
| 23 | 23354749389657604096 |
[0,1,2,6,20,80,366,1904,11080,71424,505210, 3891712,32433180,290787328,2791053734,28556359680, 310264194320,3567710830592,43287834157938, 552688817143808,7407423764750500, 103981459115606016,1525675236649033822, 23354749389657604096] |